With the recent globalization and the resulting income growth in developing countries as well as the worldwide increase in health consciousness and the resulting change in eating habits, the demand for marine products has been significantly increasing. In order to meet this demand, maritime countries have used the provisions of various treaties to ensure their exclusive access to marine products. In addition, due to overfishing by pelagic fisheries in international waters, supply of migratory fish has been steadily decreasing. Under these circumstances, not only our country but also other countries are encouraging artificial production of seedlings and various types of aquaculture of a variety of marine products.
A wide variety of marine species can be applied to aquaculture. For example, in the case of eels, juvenile eels to be cultured are collected in the spawning area of the sea and then reared until they grow to adults. In the case of tuna and red sea bream, their hatched larvae produced by fertilization are reared until they grow to adults (full cycle aquaculture system). Furthermore, in the case of marine invertebrates such as sea cucumbers, sea urchins, and shellfish (including oysters, abalones, and scallops), their larvae produced by fertilization are reared until they grow to a size large enough to be released, and then released and let them grow on their own (culture and release of artificially produced seedlings (for marine stock enhancement)).
In the case of fish, larvae hatched from fertilized eggs are much bigger than those of shellfish and thus not susceptible to feeding damage from harmful aquatic organisms. However, as described above, larvae of shellfish are susceptible to feeding damage from harmful aquatic organisms with a total length of 100 μm or less, for example. Needless to say, not only marine invertebrates as mentioned above but also brackish water or fresh water invertebrates can be applied to aquaculture.
There are various aquaculture systems. Examples of the systems include: “marine cage aquaculture” in which a net is fitted across a marine cage and juveniles are placed into the cage and cultured; and “land-based aquaculture” in which land-based tanks are placed and fish and shellfish are reared in the tanks.
Land-based aquaculture (aquaculture of fish) includes “closed recirculating aquaculture system (Patent Literature 1)” in which culture water used for aquaculture is recycled by discharging the water from a culture tank and removing or decomposing feeds and organic excrement of a fish species to be cultured, such as feces and ammonia, contained in the culture water by various mechanisms for circulation and recycling; and “flow-through aquaculture system (FIGS. 17 and 18)” in which seawater is pumped up using a water intake pump, aquatic organisms harmful to a fish species to be cultured are previously subjected to chemical treatment in a pretreatment tank, the treated water is fed to the culture tank, and water in an amount equal to the amount of the treated water thus fed is discharged from the culture tank to the sea (or a river). The most suitable aquaculture system is adopted in view of costs, work environments, and fish species to be cultured.
In the case of “flow-through aquaculture system”, a water pumping pump P1 is activated to pump water R from the sea (or a river), the pumped water R is roughly filtered through a primary filter 41 and filtered through a secondary filter 21 to remove fine contaminants, and then fed to a culture tank 10. This filtered water R contains harmful aquatic organisms 9 (such as copepods (i.e., a typical type of planktonic animals which are found throughout the year, particularly aquatic copepod crustaceans)) which may cause various problems. For example, such harmful aquatic organisms 9 may prey on or eat up larvae, or may consume feed given to juveniles metamorphosed from larvae by aquaculture producers to rear the juveniles and thus cause a problem of “feeding competition”. Therefore, the pumped seawater R must be treated to eradicate these organisms as much as possible before feeding the water R to the culture tank 10.
(It should be noted that if the species to be cultured grow to juveniles having resistance to the harmful aquatic organisms 9, they are not exposed to predatory feeding damage from the harmful aquatic organisms 9 such as copepods, but the above-mentioned “feeding competition” problem still remains unsolved.)
However, in the seawater R, the harmful aquatic organisms 9 are present not only in the form of adults but also in the form of cysts (eggs of approximately 10 μm or more). If the culture tank 10 is contaminated with these cysts, they hatch in the culture tank 10, grow vigorously in a short period of time, and cause the above-mentioned problems. In addition, ultraviolet sterilizers, ozone, etc. are not effective against such cysts. Filtration through a high-efficiency filter is the only method to solve these problems, but the use of this method is practically difficult because the filter mesh is too fine and becomes clogged in a short time.
Thus, the processes as shown in FIG. 18 are performed: a pretreatment tank 15 with a size comparable to that of the culture tank 10 is additionally prepared and a large amount of disinfectant chemical is put into the tank 15 so as to previously kill harmful aquatic organisms with high probability; then a neutralizer is added to sufficiently neutralize the disinfectant chemical; and finally the treated seawater R is drawn from the pretreatment tank 15 using a transfer pump P2 so as to feed the water R to the culture tank 10 through a transfer pipe 35.
In the existing “flow-through aquaculture” facilities configured as described above, the facility cost is much lower than that in the traditional “closed recirculating aquaculture” facilities. However, the “flow-through aquaculture” for marine invertebrates requires a large pretreatment tank 15, a large amount of disinfectant chemical and neutralizer, and constant monitoring and control of harmful aquatic organisms, and thus is the most effort-, time-, and cost-consuming system.
In addition, due to concerns about residual chemicals, there has been a demand for treatment without chemicals.
The same holds true for the above-mentioned ballast water. These days, due to globalization and the resulting rapid expansion of trading and global operation of large ships, increasing attention has been paid to a problem in which a huge amount of ballast water (water charged into a ship and discharged therefrom at each port of call to keep the unloaded ship steady) is transferred around the world and destroys the gulf ecosystems of the ports of call. Therefore, the International Convention for the Control and Management of Ships' Ballast Water and Sediments was adopted. Under the Convention, all ships are required to comply with the ballast water treatment standards (water quality not containing high levels of residual chemicals) according to the capacity of ballast water of each ship.
Under these circumstances, a device with slit openings as shown in Patent Literature 2 has been suggested as a device for use in eradicating aquatic organisms contained in seawater. In this device, however, when the seawater passes through the slit openings, a vortex is formed in front of each slit plate and a significant pressure drop occurs. Despite such a significant pressure drop, many of the aquatic organisms pass through the slit openings, which means that these organisms cannot be eradicated with high efficiency. In addition, the spacing between the slit openings cannot be reduced because the smaller spacing between the slit openings reduces the strength of the narrower portion therebetween, and thus the resulting device has a large size. The use of conventional venturi tubes also has the same problem.